Field of the Invention
The present invention relates to processes for obtaining a lipid from a cell by lysing the cell, raising a pH of the cell and/or contacting the cell with a salt, and separating the lipid. The present invention is also directed to lipids prepared by the processes of the present invention. The present invention is also directed to microbial lipids having a particular anisidine value, peroxide value, and/or phosphorus content.
Background Art
A typical process for obtaining lipids from a microbial cell, such as polyunsaturated fatty acids, involves growing microorganisms that are capable of producing the desired lipid in a fermentor, pond or bioreactor, separating the fermentation broth comprising a microbial cell biomass, drying the microbial cell biomass, and separating the lipids by solvent extraction. Steps in the separation can include diluting a fermentation broth with water, centrifuging the diluted broth, lysing the microbial cells, and extracting an intracellular lipid from the lysed cells by adding a water-immiscible solvent to the mixture in which the lipid is soluble, e.g., hexane.
Another method of extraction to remove a lipid from a microbial cell is lysing a cell in a fermentation broth using mechanical force (e.g., homogenization), enzymatic treatment, or chemical treatment to disrupt the cell walls. Lipid can be extracted from the resulting composition comprising lipids, microbial cell biomass, and water using an organic solvent, e.g., isopropyl alcohol. The lipid can be separated mechanically from the composition and the alcohol must be removed from both the lipid and the aqueous biomass waste stream. See, e.g., International Pub. Nos. WO 01/76385 and WO 01/76715.
However, industrial scale production of lipids using either of the above processes requires a large amount of volatile and flammable organic solvent, thereby creating hazardous operating conditions. The use of organic solvents in the extraction process can also necessitate using an explosion-proof lipid recovery system, thereby adding to the cost of lipid recovery. Moreover, use of an organic solvent in extracting lipid from a microbial cell can generate an organic solvent waste stream that requires a complete solvent recovery system or a proper method of disposal, which further increases the overall production cost of lipid extraction. For example, strict limits on volatile organic compound (VOC) emissions require greater manpower and added cost to vessels and other equipment.
Therefore, there is a need for a process for obtaining lipids from a cell which does not use an organic solvent. Several processes have been proposed for separating a lipid from a cell without the use of an organic solvent. For example, U.S. Pat. No. 6,750,048 discloses an aqueous washing process whereby an emulsion is washed with aqueous washing solutions until a substantially non-emulsified lipid is obtained. However, in some embodiments, this process requires multiple washing steps, which require substantial cost and time. U.S. Pat. No. 7,431,952 discloses a process whereby lysed cells are centrifuged to remove cell wall debris and then oils are extracted and purified. However, this process provides a crude oil that requires extensive further purification. Thus, what is needed is a process that does not utilize a volatile solvent to extract a lipid from a cell, and which can be performed using readily available equipment and a minimum number of steps to provide a highly pure lipid.